A printer of this type has been described in U.S. Pat. No. 6,779,867 B2.
The purpose of the spitting pulses is to prevent nozzle failures or nozzle malfunction that might be caused when the ink tends to dry out in the nozzle orifice while the nozzles are not used for a certain time. For each nozzle, the spitting pulses are timed such that another ink droplet is ejected before the ink has had time enough to dry out to such an extent that solidified ink sticks firmly to the wall of the nozzle orifice. Then, the droplet being ejected will remove the dried ink and clear the nozzle orifice again.
Spitting pulses may have to be applied regularly and hence, they may have to be applied onto the print medium at regular intervals. Because the spitting pulses may have to be applied at regular intervals, the resulting spit-droplets may have to be applied without taking into account the image to be printed. Therefore, spit-droplets may be applied onto a position of the recording medium which, pursuant to the print instructions, should not receive any ink. Although the individual ink dots are relatively small and spitting is controlled such that isolated drops will be distributed quasi-randomly over the media sheet, frequent spitting may degrade the quality of the printed image.
If the image is printed using a plurality of colors, spitting may result in droplets of the “wrong color” being applied onto the print medium. For example, when a yellow area is printed pursuant to print instructions, then a black spitted droplet spitted in that yellow area may decrease the print quality.
U.S. Pat. No. 6,508,528 B2 proposes an alternative approach for coping with the problem of ink drying out in the nozzle orifices. Instead of actually spitting ink droplets onto the recording medium, the actuators are exited by so-called pre-fire pulses the amplitude of which is kept so small that the meniscus of the liquid ink is only vibrated in the nozzle orifice but no droplets are formed and ejected. The vibrations induced in the liquid ink have the purpose to remove or dissolve the dry ink that would otherwise adhere to the walls of the nozzle orifices. However, in order to be effective, it is necessary to apply several hundreds or several thousands of pre-fire pulses to each nozzle before this nozzle is used again for printing. The large number of pre-fire pulses therefore implies an increased heat dissipation and energy consumption and may also reduce the life time of the actuators.
It is an object of the invention to provide an ink jet printer which can achieve an improved print quality without increased energy consumption or accelerated ageing of the print head.